I connected the rear shelf speakers using the Alpine KTP-445U. Here is my description.
Before you start buying equipment, use a flashlight and look through the rear deck grill to confirm that there are speakers installed. Or check "software -> Additional vehicle information" - it should say "Audio system: Premium speakers, no external amplifier".
Supplies:
⦁ Alpine KTP-445U 4-Channel Amp
⦁ 18 AWG speaker wire - approx. 15 ft. (I had some 16 gauge so that is what I used.)
⦁ 18 AWG T-taps and male connectors - 8 pairs (Note: I've seen pictures of people using the red 18 AWG T-taps and others using the blue 16 AWG. The speaker wires in the amp's output harness are 18 AWG and it looks like the wires in the car are 18 although I couldn't find printing on them.)
⦁ gray twist on wire connectors - 4
⦁ 5/16" stud 16 AWG ring terminals - 2 (the power wire is 16 AWG not 18)
⦁ For the connectors I recommend
Wire Terminals and Connectors - WiringProducts. Ltd.
⦁ 10mm wrench or socket
⦁ 14mm socket
⦁ wire cutter, wire stripper, crimping tool, pliers
⦁ trim removal tools
⦁ head lamp and flashlight
⦁ short stool - very helpful to sit on when working on the wires inside the B-pillar
⦁ I used some wood blocks to hold the B-pillar open while I was working. It was a tight area to work in.
Consider unplugging the charging cable so it isn't in your way when you are walking around. I am not going to describe removing the back seat and panels since that is covered elsewhere. My notes start after you have access to the wires.
Here is a good thorough video on using connectors and crimping tools:
Here is a video on using T-taps:
This Crutchfield video gives you an overview of the amp's wires starting at 2:16
At 3:21 begins an explanation that the input blue wire isn't needed with a factory audio system.
1. Decide which side of your speaker wire will be positive and make sure it is marked clearly so you can be consistent.
2. Open all 4 windows, all 4 doors and the frunk. You won't be able to open them with the power off.
3. Power off the car by going to "Safety & Security" and selecting "POWER OFF".
4. Disconnect the negative side of the 12v battery in the frunk (10mm wrench or socket. See link in step 5 for pictures) - don't close the frunk!
5. Open the contactor in the penthouse. Here is a good description:
Tesla Model 3 Hard Reset | Mountain Pass Performance
6. From the back side of the left B pillar push the end of your speaker wire through to the front. In the back unspool wire to where you will place the amp giving yourself some slack and then cut the wire. Do this again on the right side.
7. At the front of the left B-pillar T-tap the red + wire and connect it to the + side of your wire, T-tap the black - wire and connect it to the - side of your wire.
8. At the front of the right B-pillar T-tap the blue + wire and connect it to the + side of your wire. T-tap the green - wire and connect it to the - side of your wire.
9. In the back run the left side speaker wire to the right side. I ran mine under the edge of the carpet under the back seat.
10. Cut the RCA connectors off of the Alpine "REAR IN" wires. (green and purple. For reference when stripping - these are 20 AWG.)
11. Use the gray twist on connectors to connect your left side + wire to the green + Alpine input wire and your left side - wire to the green/black - Alpine input wire.
12. Use the gray twist on connectors to connect your right side + wire to the violet + Alpine input wire and your right side - wire to the violet/black - Alpine input wire.
13. At the speaker connector behind the right bolster near the top, T-tap the purple + wire and connect it to the green + Alpine output wire and T-tap the white - wire and connect it to the green/black - Alpine output wire.
14. At the same speaker connector , T-tap the gray + wire and connect it to the violet + Alpine output wire and T-tap the yellow - wire and connect it to the violet/black - Alpine output wire.
15. Set the amp's input configuration dip switch to 4CH and the input signal type to SPKR. Leave the other switches where they are. The video linked below in step 23 shows setting the dip switches at 9:15.
16. Connect the input and output wiring harness plugs to the Alpine amp.
17. Connect one 5/16" stud 16 AWG ring terminal to the amp's yellow wire.
18. Connect one 5/16" stud 16 AWG ring terminal to the amp's black wire.
19. Connect the yellow wire to the positive (red) post in the penthouse under the back seat (14mm socket).
20. Connect the black wire to the post for the ground (14mm socket).
21. Close the contactor and put the foam cover back over the penthouse.
22. Reconnect your 12v battery in the frunk and power the car on.
23. Follow the instructions for setting the rear gain. This is described in the instructions that came with the amp and also in this video at 20:48.
To help with this use camp mode so the car will not turn the audio off when you move from the front to the back to adjust the setting. Leave the rear window open so you can adjust the gain without opening the door.
24. Smile!
Other people have mounted the amp behind the right rear bolster and that is what I did. Since removing the rear bolster was hard and I don't want to risk breaking the clips, I haven't put it back yet in case I need to adjust things. Remember to reconnect the wires under the back seat at each side and pull the seat belt buckles up when you put the seat in place.
